1. Field of the Invention
The present invention relates to a cylindrical alkaline storage battery, and, more particularly, to a cylindrical alkaline storage battery suitable for achieving a high capacity.
2. Description of the Related Art
Alkaline batteries available include, for example, a nickel cadmium secondary battery and a nickel hydrogen secondary battery depending on the types of active materials contained in the batteries. Some of those alkaline batteries are of a cylindrical type which has a cylindrical case. The case is sealed with a lid having a relief valve and accommodates an electrode assembly together with an alkaline electrolyte. The electrode assembly is formed by winding a belt-like negative plate and a belt-like positive plate spirally with a separator in between and are contained in the case while that part of the negative plate which is wound around the outermost one of the electrode assembly contacts the inner wall of the case.
The positive plate, which is called a nickel electrode, is formed by filling a positive mixture in a nickel porous member having a three-dimensional mesh structure. The positive mixture includes nickel hydroxide particles as a positive active material, additive particles and a binder which binds those particles. The negative plate is formed by covering both sides of a metal sheet as a negative substrate with a hydrogen absorbing alloy layer as a negative active material layer. The metal sheet has through holes in which the negative active material is filled. The hydrogen absorbing alloy layer and a filler includes hydrogen absorbing alloy particles which can absorb and desorb hydrogen as a negative active material, and a binder which binds the hydrogen absorbing alloy particles. While the capacity of each of the positive plate and the negative plate is defined by the amount of the active material or the amount of the hydrogen absorbing alloy contained therein, the battery capacity is defined by the capacity of the positive electrode. This is because the capacity of the negative electrode in this type of cylindrical alkaline storage battery is set greater than the capacity of the positive electrode in order to prevent the inner pressure from rising by reducing an oxygen gas produced in the positive plate with the negative plate when the battery is overcharged.
Recently, there have been strong demands of a higher capacity or an improvement on the volume energy density for cylindrical alkaline storage batteries of this type, particularly, cylindrical alkaline storage batteries of AA size compatible with AA-size dry cells. To increase the battery capacity, the capacity of the positive electrode should be increased. Specifically, the amount of the positive active material should be increased or the ratio of usage of the positive active material should be improved. One known way to increase the amount of the positive active material is to increase the length, thickness and area of the positive plate and the filling density of the positive mixture in the porous member. For example, Japanese Patent Laid-Open Publication No. Hei 10-199520 discloses a cylindrical alkaline storage battery which achieves a high capacity by setting the thickness of a nickel electrode equal to or greater than 0.8 mm.
However, the service life of the battery becomes shorter when the cylindrical alkaline storage battery described in the publication is adapted to an AA-size cylindrical alkaline storage battery whose case has an outside diameter of 13.5 mm to as large as 14.5 mm and whose positive plate is made as thick as 0.95 mm or thicker for a higher capacity so that the volume energy density becomes 340 Wh/l or higher.
Before going into the detailed description of the problem, definitions of terms to be used hereinafter will be given below.
Capacity ratio: the ratio of the capacity of the entire negative plate to the capacity of the positive electrode
Non-overlapping portion: the portion of the negative active material layer which does not overlap the positive plate via the separator
Overlapping portion: the portion of the negative active material layer which overlaps the adjoining positive plate via the separator
Ratio of the non-opposing portion of the negative plate: the occupying ratio of the amount of the negative active material contained in the non-overlapping portion of the negative active material layer and the filler distributed in an area of the negative substrate that is covered by the non-overlapping portion on both sides thereof to the total amount of the negative active material
Opposing capacity ratio: the ratio of the capacity of the overlapping portion and the filler distributed in an area of the negative substrate that is covered by the overlapping portion on at least one side thereof in the negative plate and the through holes to the capacity of the positive electrode
Capacity-electrolyte ratio: the ratio of the volume of the alkaline electrolyte to 0.2 C capacity
As the capacity of the positive electrode is increased by making the positive plate thicker, the capacity ratio decreases, thereby reducing the amount of the negative active material contained in the overlapping portion of the negative active material layer (hydrogen absorbing alloy layer). As the battery reaction mainly progresses between the positive active material and the overlapping portion of the negative active material layer at the time of charge/discharge, the battery reaction does not progress smoothly if the amount of the negative active material contained in the overlapping portion is small.
In a battery with the ratio of the non-opposing portion of the negative plate of 29%, for example, when the capacity ratio drops to 1.4 or lower, the opposing capacity ratio becomes 1.00 or less, so that the capacity of the negative electrode substantially becomes smaller than the capacity of the positive electrode.
When the opposing capacity ratio becomes 1.00 or less, it becomes impossible to exchange protons at the shortest distance at the time the battery reaction occurs. So the reaction is made non-uniform, thereby lowering the discharge characteristics. Further, the time for the oxygen gas that has been produced in the positive plate at the time of charging to pass through the separator, reach the negative plate and be reduced becomes longer, thus increasing the inner pressure of the battery. This actuates the relief valve, so that the alkaline electrolyte leaks out. When charge/discharge is repeated, therefore, the battery life becomes shorter due to two factors: early and local deterioration of the active material caused by the non-uniform reaction and the leakage of the alkaline electrolyte caused by an increase in the inner pressure of the battery.
In addition, when the capacity of the positive plate or the positive capacity increases, the capacity-electrolyte ratio drops. When the capacity-electrolyte ratio becomes 0.85 ml/Ah or less, the amount of the electrolyte becomes short at the portion where the positive plate and the negative plate overlap each other via the separator. This increases the electric resistance, lowering the discharge characteristic.
Because the alkaline electrolyte mainly exist in the form of being contained in the positive plate, the negative plate and the separator in entirety in the battery, some of the alkaline electrolyte is contained in the non-overlapping portion of the negative plate which does not directly contribute to the battery reaction and the separator adjoining to the non-overlapping portion. Accordingly, the amount of the alkaline electrolyte which is contained in the positive plate, the overlapping portion of the negative plate where the battery reaction takes place, and the separator sandwiched therebetween is what is obtained by subtracting the amount of the alkaline electrolyte contained in the non-overlapping portion of the negative plate from the total amount of the alkaline electrolyte. In the case where the capacity-electrolyte ratio becomes 0.85 ml/Ah or less, if a part of the alkaline electrolyte is contained in the non-overlapping portion of the negative plate, the amount of the alkaline electrolyte that is present in the place of the battery reaction becomes short. This increases the electric resistance between the positive plate and the negative plate, thus lowering the discharge characteristic.
When continuous charging takes place at a low temperature, the positive plate is expanded to absorb the electrolyte. In a battery which has a small amount of the electrolyte, therefore, the discharge performance after continuous charging drops, causing a significant voltage drop at the initial discharge stage. This low-temperature continuous charge characteristic also considerably decreases when the capacity-electrolyte ratio becomes 0.85 ml/Ah or less.